When Ben Askren retired from MMA last year as the ONE welterweight champion, he didn't completely rule out an eventual return to the mat. The UFC was on his mind, especially for the right opponent, but he wasn't optimistic it was in the cards.

That changed in the past week as the UFC and ONE Championship orchestrated a "trade," releasing Askren and longtime flyweight champion Demetrious Johnson to go to the other promotion. A move between organizations like this has never occurred in the sport's history.

"I thought of a lot of different scenarios in which I was allowed to fight the best in the world," Askren told ESPN's Ariel Helwani on Monday. "Even in my crazy head, there was no scenario where I got traded to do this. The UFC sees the value for me because they traded me for the greatest fighter that has ever stepped on planet earth."

Askren, a former All-American and NCAA champion wrestler at Missouri, said he is likely to restructure his contract once officially with the UFC. He has only a short amount of time left on his current contract and believes both sides want to work out a long-term deal. Askren expects to be back in the Octagon in the early part of 2019.

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UFC has agreed to release Demetrious Johnson so he can sign with ONE Championship, which has agreed to release Ben Askren to sign with UFC.

Multiple fights means multiple opponents, and there's no shortage of options for Askren. He told Helwani his ideal first matchup is against Georges St-Pierre, the former UFC welterweight champion and arguably the best MMA fighter ever. St-Pierre, 37, has not fought since claiming the UFC's middleweight belt last November. He later vacated the title.

St-Pierre told ESPN recently he wants to return only for "big fights that have a great legacy." This could be one of them.

"If there is a chance you are coming back, you need to tell us. Fans would like to see you and me tango," Askren said of St-Pierre. "I think Georges is one of ... if not the greatest welterweight of all time. That being said, I think my skill set matches up very, very well with him. I love challenges. I want to fight Georges St-Pierre."

The other name Askren is interested in is UFC lightweight champion Khabib Nurmagomedov. If this matchup comes together, he said, it would be at a higher weight class, potentially at the hypothetical 165-pound division or a catch weight. He said he cannot get down to the 155-pound limit required for lightweight.

Helwani also asked about the possibility of facing UFC welterweight champion Tyron Woodley, Askren's former college teammate and longtime friend. Askren said it will never happen.

"Tyron and I aren't fighting. I'm not fighting Tyron Woodley," Askren said. "If say he breaks his knee or something and they want to give me the interim belt, I'll fight for that. We've been really close since I was 17 years old."

Johnson, meanwhile, is also excited to work for a new fight promotion. The former UFC flyweight champion, who defended the belt a record 11 consecutive times, said he is "extremely happy, ecstatic about this."

Part of that is for financial reasons. According to his agent, Malki Kawa, Johnson will become the highest-paid fighter at ONE Championship. This would surpass former UFC champion Eddie Alvarez, who told ESPN last week he will make at least eight figures guaranteed in his new deal with ONE.

Johnson was not a major draw for the UFC, particularly on pay-per-view cards, and as a result did not receive compensation equivalent to his overall dominance. UFC President Dana White said he hopes this changes for the better at ONE Championship.

"If you look at it, the kid broke the record for the most title defenses," White said. "I feel like he never really got the love and the respect that he deserved from the fans. I don't know if he was always thrilled being here. So hopefully he's a lot happier where he's headed now. His longtime trainer and coach [Matt Hume] is an executive over there, so maybe he'll be happier over there."

Johnson expects to make his debut just after the start of 2019. He will look to rebound from a close decision loss to Henry Cejudo in August, his first defeat in seven years.

Despite Johnson's exceptional MMA resume, he does not expect an immediate title shot.

"I have to work my way up to see if I can capture that belt and see if I can defend it," Johnson said. "That's the goal: become the champion and defend it as many times as you can. Here we go, all over again.

"At the end of the day, I'm a workhorse, I'm a workman, I'm a martial artist. There are people who are working for the same goal over there longer than I have. I don't get to come in there and strut my stuff. I'm the new kid on the block."